The present invention relates generally to diagnostic testing of a successive approximation A/D (analog-to-digital) converter, and in particular to real time self-testing of a successive approximation A/D converter when incorporated into some signal processing system.
This type of A/D converter is generally used as a basic functional block which is incorporated into larger signal processing systems. Such systems may include radio communications systems where a signal or group of signals are converted to an equivalent digital signal for subsequent digital signal processing or transmission. Self-testing capability of these A/D converters is valuable for notifying a system operator or system controller that a fault exists and that prompt replacement is required. Moreover, many systems employ "hot switchover" to redundant modules which automatically effects the clearing of the faulted module within the system to reduce system interruptions even further.
In an A/D converter of the successive approximation type, an analog input signal is tracked and then held by a track/hold circuit (also known as sample/hold). While this value is being held, a register, designated a successive approximation register (SAR), controls an internal D/A converter in such a way that the D/A converter output value converges towards the held input value. This is typically accomplished by making consecutive "guesses", observing the output of the comparator to determine if the "guess" was high or low, and then making another "guess" based on that information. After each "guess", the range of uncertainty is cut in half. Thus a 12-bit successive approximation A/D converter, for example, converges to a final result by taking 12 consecutive "guesses", or successive approximations.
In the prior art, the most common method utilized for diagnostic testing of successive approximation A/D converters is to provide capability for off-line testing. Such a method relies on injecting a known analog input level at the track-and-hold input, converting it, and then observing the digital output word for the correct result. Such methods, however, are not practical for certain applications because they require successive approximation A/D converters to perform a new convergence and therefore they require an additional complete conversion cycle to be made. Moreover, they only test a successive approximation A/D converter's ability to convert one particular input value. Thus the commonly known art related to diagnostic testing of successive approximation A/D converters suffers from relatively slow speed and incomplete evaluation of the A/D converter over the entire operational range of input voltages.
Accordingly, there exists a need for an improved diagnostic testing capability for successive approximation A/D converters which evaluates substantially all components within the A/D converter while remaining on-line, and which is not restricted to one particular reference input value.